Conventionally, positioners have been provided in regulator valves, where the degree of opening of the regulator valve is controlled by the positioner. This positioner comprises a controlling portion that calculates a difference between a valve opening setting value that is sent from the higher-level device and an actual opening value that is fed back from the regulator valve, and outputs, as a control output, an electric signal that is produced through performing a specific calculation on this difference, an electro-pneumatic converting device (EPM) that converts into an air pressure signal the control output from the controlling portion, a pilot relay for amplifying the air pressure signal, converted by the electro-pneumatic converting device, and outputting it to a driving portion for the regulator valve, and an opening sensor for detecting the degree of valve opening of the regulator valve and sending it as the actual opening value to the controlling portion (See, for example, Japanese Unexamined Utility Model Registration Application Publication S62-28118).
Conventionally positioners are designed so as to operate with an electric current between 4 and 20 mA sent through a pair of electric wires from a higher-level system. For example, if a current of 4 mA is sent from the higher-level system, the opening of the regulator valve is set to 0%, and if a current of 20 mA is sent, then the opening of the regulator valve is set to 100%.
In this case, the supplied electric current I from the higher-level system varies in the range of 4 mA (the lower limit electric current value) through 20 mA (the higher limit electric current value), and thus the electric current required in the internal circuitry within the positioner is limited to no more than the 4 mA (for example, 3.8 mA) that can always be secured as an electric current value that is supplied from the higher-level system.
The valve opening setting value for the valve is inputted into the positioner by the higher-level system. Moreover, the actual opening value for the valve is obtained through the opening sensor. Consequently, the positioner is able to perform valve fault diagnostics, self-diagnostics, and the like, through performing calculations on the relationship between the valve opening setting value and the actual opening value for the valve. The provision of such fault diagnostic functions in the positioner makes it possible to increase the functionality of the system at a low cost, through eliminating the need for providing a separate fault diagnosing device (See, for example, Japanese Unexamined Patent Application Publication 2004-151941).
For reasons such as these, in recent years positioners have been provided with communication functions, self-diagnostic functions, valve diagnostic functions, valve opening output functions, and other additional functions, in addition to the primary function (the valve control function (the basic function)) of controlling the opening of the valve.
On the other hand, in order to improve control characteristics, there have been proposals for control methods wherein control outputs to the electro-pneumatic converting device are corrected through feeding back, as a minor loop, the air pressure outputted from the pilot relay, in addition to the conventional control loop based on the difference between the valve opening setting value and the actual opening value, as disclosed in Japanese Examined Patent Application Publication 2004-523016. Moreover, as disclosed in Japanese Unexamined Patent Application Publication 2001-221201, there have been proposals for control systems for correcting the control outputs to the electro-pneumatic converting device through feeding back, as a minor loop, the dislocation of the cylinder or poppet valve in the pilot relay, in addition to the conventional control loop based on the difference between the valve opening setting value and the actual setting value. This type of control method is known as double-loop control. Note that the control method that uses only the control loop based on the difference between the valve opening setting value and the actual opening value is known as single-loop control.
However, in a conventional positioner, the additional provision of a pressure sensor for detecting the output air pressure Po of the pilot relay, a location sensor for detecting the dislocation Xp of the cylinder or poppet valve in the pilot relay, and sensors for fault diagnostics (such as a pressure sensor for detecting the supply air pressure Ps to the electro-pneumatic converting device or pilot relay, a pressure sensor for detecting the output air pressure (nozzle back pressure) Pn from the electro-pneumatic converting device to the pilot relay, an oscillation detecting sensor for detecting the oscillation G1 within the positioner, and the like), in addition to the opening sensor for detecting the opening of the regulator valve, as the result of responding to improvements in controllability and multifunctionality, as described above, while substantially increasing the functionality when compared to before, causes the electric current required in order to exhibit all of these functions simultaneously to exceed the 3.8 mA limitation, thus increasing also the susceptibility to not being able to exhibit all functions with stability.
The present invention is to solve problems such as set forth above, and the object thereof is to provide a positioner that is able to exhibit high functionality reliably, while avoiding an inadequate electric current supply.